Selective filtering of wavelength-converted semiconductor light emitting devices

ABSTRACT

A structure includes a semiconductor light emitting device including a light emitting layer disposed between an n-type region and a p-type region. The light emitting layer emits first light of a first peak wavelength. A wavelength-converting material that absorbs the first light and emits second light of a second peak wavelength is disposed in the path of the first light. A filter material that transmits a portion of the first light and absorbs or reflects a portion of the first light is disposed over the wavelength-converting material.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/260,090filed Sep. 27, 2002 now U.S. Pat. No. 6,744,077 and incorporated hereinby reference.

BACKGROUND

1. Field of Invention

This invention relates to semiconductor light emitting devices includinga wavelength-converting material and a filter material.

2. Description of Related Art

The color of light emitted from a semiconductor light emitting devicechip such as a light emitting diode may be altered by placing awavelength-converting material in the path of the light exiting thechip. The wavelength-converting material may be, for example, aphosphor. Phosphors are luminescent materials that can absorb anexcitation energy (usually radiation energy) and store this energy for aperiod of time. The stored energy is then emitted as radiation of adifferent energy than the initial excitation energy. For example,“down-conversion” refers to a situation where the emitted radiation hasless quantum energy than the initial excitation radiation. The energywavelength effectively increases, shifting the color of the lighttowards red.

If some light emitted from the chip is not absorbed by the phosphor, theunconverted light emitted from the chip mixes with the light emittedfrom the phosphor, producing a color between the color of the lightemitted from the chip and the color of the light emitted from thephosphor. When used in applications requiring a particular color, thecolor of light emitted from the chip and the amount of light convertedby the phosphor must be tightly controlled.

SUMMARY

In accordance with embodiments of the invention, a structure includes asemiconductor light emitting device including a light emitting layerdisposed between an n-type region and a p-type region. The lightemitting layer emits first light of a first peak wavelength. Awavelength-converting material that absorbs the first light and emitssecond light of a second peak wavelength is disposed in the path of thefirst light. A filter material that transmits a portion of the firstlight and absorbs or reflects a portion of the first light is disposedover the wavelength-converting material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the present invention.

FIG. 2 illustrates an alternate embodiment of the present invention.

FIG. 3 illustrates the emission spectra of two devices, one with afilter layer and one without a filter layer.

FIG. 4 illustrates the lumen output of several phosphor converted lightemitting devices as a function of fraction of light unconverted by thephosphor layer.

FIG. 5 is an exploded view of a packaged light emitting device.

FIG. 6 is a plot of extraction efficiency as a function of pump leakagefraction for a wavelength-converted light emitting device.

FIG. 7 is a plot of conversion efficiency as a function of pump leakagefraction for blue and UV wavelength-converted devices without filters,with absorbing filters, and with reflectors.

DETAILED DESCRIPTION

In accordance with embodiments of the invention, a light emitting deviceincludes a wavelength-converting layer for converting the wavelength oflight emitted from a light emitting device chip, and a filter layer forfiltering out any unconverted light from the chip. The example of aIII-nitride flip chip is considered below. It is to be understood thatthe invention is not limited to the materials, device orientations, orother details discussed in the examples below. For example, theembodiments of the invention may be applied to any suitable lightemitting device materials system, including for example III–V materials,III-nitride materials, III-phosphide materials, and II–VI materials.Embodiments of the invention may be applied to any device geometry,including devices with contacts on opposite sides of the semiconductorlayers and devices with contacts on the same side of the semiconductorlayers, such as flip chips where light is extracted through a substrate,and epitaxy-up structures where light is extracted through the contacts.

FIG. 1 illustrates an embodiment of the present invention. A lightemitting device chip includes a substrate 10, an n-type region 12, anactive region 14, and a p-type region 15. In one embodiment, n-typeregion 12, active region 14, and p-type region 15 are III-nitridematerials, having the formula Al_(x)In_(y)Ga_(z)N, where 0≦x≦1, 0≦y≦1,0≦z≦1, x+y+z=1. Substrates suitable for the growth of III-nitridematerials include GaN, SiC, and sapphire. Each of n-type region 12,active region 14, and p-type region 15 may be a single layer or multiplelayers with the same or different compositions, thicknesses, and dopantconcentrations. A portion of p-type region 15 and active region 14 isremoved to expose a portion of n-type region 12. Contacts 16 are formedon the remaining portion of p-type region 15 and the exposed portion ofn-type region 12. Contacts 16 may be electrically and physicallyconnected to a submount 18 by submount interconnects 17. Submountinterconnects may be, for example, solder. Contacts 16 may bereflective, such that light generated in active region 14 is extractedfrom the chip through substrate 10.

A wavelength-converting layer 20 is formed over the chip. Thewavelength-converting material may be, for example, yttrium aluminumgarnet doped with praseodymium and cerium (YAG:Pr+Ce), strontium sulfidedoped with europium (SrS:Eu), strontium thiogallate, or any othersuitable phosphor. Wavelength-converting layer 20 may be formed by, forexample, electrophoretic deposition, stenciling, screen printing, andany other suitable technique. Wavelength-converting layer 20 need notcover all of the top and sides of the chip, as illustrated in FIG. 1.Multiple wavelength-converting materials capable of converting thewavelength emitted from the chip to the same or different wavelengthsmay be incorporated into wavelength-converting layer 20, orwavelength-converting layer may be comprised of multiple discretesublayers, each containing a different wavelength-converting material.

A filter layer 19 is formed over wavelength-converting layer 20. Filterlayer 19 absorbs light of the wavelength emitted by active region 14 ofthe light emitting device chip and transmits light of the wavelength orwavelengths emitted by wavelength-converting layer 20. The materials infilter layer 19 may be selected and deposited such that some or all ofthe unconverted light emitted by the chip is prevented from escaping thedevice. The filter material may be, for example, a material that absorbsthe unconverted light emitted by the chip and dissipates the energy asheat. Examples of suitable filter materials include inorganic andorganic dyes.

FIG. 2 illustrates an alternative embodiment of the present invention.In the embodiment illustrated in FIG. 2, rather than being conformallycoated on the light emitting device chip, the filter layer andwavelength-converting layer are dispersed in a material overlying thechip. In the device illustrated in FIG. 2, a pedestal 25 supports lightemitting device chip 24 and lens 22, creating a space between chip 24and lens 22 that may be occupied by a hard or soft encapsulant. Examplesof suitable encapsulant materials include hard or soft silicone andepoxy. The encapsulant material is generally selected such that therefractive index of the encapsulant matches the refractive index ofmaterials adjacent to the encapsulant (e.g. the substrate of the chip)as closely as possible. In addition, the encapsulant material may beselected for its ability to mix with the wavelength converting materialand/or the filter material. Particles of a wavelength-convertingmaterial such as phosphor are dispersed in a first layer of encapsulant26 close to chip 24. Particles of a filter material are dispersed in asecond layer of encapsulant 28 overlying wavelength-converting layer 26.The same or different encapsulating materials may be used for thewavelength-converting material and the filter material. Chip 24,wavelength-converting layer 26, filter layer 28, and lens 22 need not beadjacent to each other as illustrated in FIG. 2. Air, additional layersof encapsulant, or layers of other materials may separate any of thelayers illustrated in FIG. 2. In addition, wavelength-converting layer26 may contain multiple wavelength-converting materials, or may bemultiple discrete sublayers containing the same or differentwavelength-converting materials. Filter layer 28 may also containmultiple filtering materials and may be multiple discrete sublayerscontaining the same or different filtering materials.

In other embodiments, filter layer 28 of FIG. 2, which includes a filtermaterial dispersed in an encapsulant, is used in combination with theconformal wavelength-converting layer 20 of FIG. 1. In otherembodiments, the filter material may be coated on the inside of lens 22of FIG. 2, on the outside of lens 22, or incorporated in the materialthat forms lens 22. In the embodiments described above, the filtermaterial absorbs the unwanted unconverted emission from the chip. Instill other embodiments, the filter material may reflect the unwantedunconverted emission from the chip, while transmitting the wantedwavelength-converted emission. For example, the filter material may be aseries of layers of dielectric materials with refractive indicesselected to transmit the wavelength converted light while reflecting theunconverted light. The dielectric materials may be selected from knowncoatings for the ability to transmit the converted wavelengths andreflect the unconverted wavelengths, and the ability to withstand boththe converted and unconverted wavelengths.

In one example, the unconverted emission from the chip is blue lightwith a wavelength less than 500 nm and the wavelength-converted emissionis green with a wavelength greater than 500 nm.

FIG. 3 illustrates the emission spectra of two devices, each with a 450nm blue-emitting chip covered and a 535 nm green-emitting phosphorlayer, one with a filter layer (device A) and one without (device B).The inclusion of a filter layer in device A almost completely preventslight emitted by the device chip from escaping the chip. Thus, lightfrom the device chip does not significantly impact the color of lightvisible from device A. The light from device A will appear green. Incontrast, device B has a small peak of emission from the device chip.The light from device B may appear greenish blue due to the emissionfrom the chip.

Including a filter layer in a wavelength-converted semiconductor lightemitting device may offer several advantages. First, the use of a filterlayer allows tight control of the color and color purity of lightproduced by the device. The wavelength of light emitted by a lightemitting device chip depends on the composition of the active region,which may be difficult to control during fabrication. In contrast,typically wavelength-converting materials emit the same color of highcolor purity light regardless of the wavelength of the absorbed light,as long as the wavelength of the absorbed light is in a wavelength rangecapable of exciting the wavelength-converting material. Accordingly, theuse of a wavelength-converting material improves the uniformity of thecolor of light produced by various devices. The uniformity acrossdevices can be compromised if light from the chip is permitted to mixwith the wavelength-converted light. The use of a filter layer preventslight from the chip from escaping the device, thus the only lightescaping from the device is the high color purity light emitted by thewavelength-converting material.

Second, the use of a filter layer may increase the lumen output of awavelength converted semiconductor light emitting device. FIG. 4illustrates the lumen output of several devices as a function of thefraction of light emitted by the device chip that is unconverted by aphosphor. As illustrated in FIG. 4, devices that allow some lightemitted from the chip to leak through the phosphor layer unconvertedexhibit a higher lumen output than devices that phosphor-convert all ofthe light emitted from the chip. In order to completely convert alllight emitted by the device chip, a thick phosphor layer must be used.The thick phosphor layer may result in increased back scattering oflight, which increases the likelihood that light will be lost throughabsorption by semiconductor layers in the chip or other portions of thedevice, reducing the total lumen output of the device. In applicationswhere mixing of the light emitted by the chip and the light emitted bythe phosphor is undesirable, the use of a filter permits tuning thethickness and other characteristics of the phosphor layer for maximalphosphor emission, while maintaining the color and color purity ofemission from the phosphor by selective absorption of the unconvertedlight from the chip.

Filters can also be employed to improve the efficiency of LEDs for thegeneration of white light. This opportunity arises because of the strongdependence of chip extraction efficiency on the loading density or totalthickness of wavelength-converting particles surrounding the chip, asdescribed above. As the loading density is increased, the extractionefficiency is reduced. This effect is most easily observed by measuringthe light generation efficiency of the device as a function of pumplight leakage, which is that fraction of light which is emitted directlyfrom the chip compared to the total amount of generated light (pumplight plus converted light). Such measurements have been performed andresult in data similar to that shown in FIG. 6.

For an LED chip employing wavelength-conversion media, the total lightgenerated may be written asΦ_(pump)+Φ_(conv)=Φ_(in) QE QD C _(ext)where Φ_(pump) is the output flux from direct chip emission (i.e.,leaking through the converting media), Φ_(conv) is thewavelength-converted output flux, QE is the quantum efficiency of thewavelength converter, QD is the associated quantum deficit in photonenergy, and C_(ext) is the extraction efficiency which depends on pumpleakage fraction, which may be writtenF_(pump)=Φ_(pump)/(Φ_(pump)+Φ_(conv)).

The converted light output can now be written in terms of the pumpleakage fraction, so thatΦ_(conv)=(1−F _(pump))Φ_(in) QE QD C _(ext)This expression can be used to determine the relative conversionefficiency of chips employing wavelength-converting media as a functionof pump leakage fraction, using the experimental dependence given byFIG. 6. Using this approach, we are able to compare the examples ofgenerating white light using either a blue pump LED chip (where the blueemission contributes directly to the white spectrum) or a UV-based pump(wherein the light does not contribute to useable spectrum). The resultsare shown in FIG. 7, which illustrates calculated conversionefficiencies of LEDs employing wavelength converting media, using eitherblue or UV pump wavelengths. The conversion efficiency is defined as thetotal emitted power for the device employing the wavelength converter,divided by the total emitted power of the bare pump LED (no wavelengthconverting media). The curves labeled “filtered” are data for the caseof the device employing an absorbing filter mechanism to reduce oreliminate the leaked pump light as required to maintain the desiredoutput spectrum requirements. The curves labeled “reflector” are datafor the case of the device employing a reflective material to reflectleaked pump light back into the device.

These calculations assume wavelength converters of peak wavelengths at450 nm (blue), 540 nm (green) and 620 nm (red), and with spectral widthstypical for phosphors. The blue pump wavelength was taken as 450 nm,while that of the UV pump was taken as 390 nm. For the cases wherefilters or reflectors are employed, the filter/reflector insertion lossis taken as 10%. The combination of blue, green, and red light was keptat a radiometric ratio of 8%, 37%, and 55% as an estimate to target2900K white light.

As a reference point, we note the case of a UV-based pump wherein allthe light is converted (0% pump leakage, which is presumably necessaryfor eye safety reasons) gives the poorest conversion efficiency at 12%.A blue-based pump, allowing˜8% leakage which is directly used in thefinal spectrum, gives a much higher conversion efficiency of˜38% (morethan a factor of three improvement).

A dramatic improvement in conversion efficiency can be obtained byallowing more pump leakage, especially for the UV pump case. Byemploying a filter which later blocks out all the UV light, the devicealloying an initial 20% leakage achieves a conversion efficiency of31.5%, more than a factor of two improvement over the “100% converted”case. A similar, although much weaker, improvement for the blue pumpcase is obtained by allowing 20% leakage, and employing a filter tocorrect the final white color point. For this case the conversionefficiency increases from 38% to 39%.

The use of a reflector rather than an absorbing filter material furtherimproves conversion efficiency. In the case of a UV pump device allowing20% leakage, the use of a reflector rather than an absorbing filterincreases the conversion efficiency from 31.5% to 34%. In the case of ablue pump device allowing 20% leakage, the use of a reflector ratherthan an absorbing filter increases the conversion efficiency from 39% to42%.

It is clear from this work that allowing more pump light leakage andcorrecting the final spectra with absorbing or reflecting filters ismost important for cases where low pump leakage is required, either inthe blue pump case where low white color temperature are required, or inthe UV pump case (all cases).

FIG. 5 is an exploded view of a packaged light emitting device. Aheat-sinking slug 100 is placed into an insert-molded leadframe 106. Theinsert-molded leadframe 106 is, for example, a filled plastic materialmolded around a metal frame that provides an electrical path. Slug 100may include an optional reflector cup 102. Alternatively, slug 100 mayprovide a pedestal without a reflector cup. The light emitting devicedie 104, which may be any of the devices described above, is mounteddirectly or indirectly via a thermally conducting submount 103 to slug100. An optical lens 108 may be added. In embodiments where thewavelength-converting material and/or filter material are dispersed inencapsulants, the encapsulants may be injected between die 104 and lens108. In embodiments, where a reflector is used to reflect unwanted pumpemission back into the device, the reflector may be a series ofdielectric layers applied to the inside or the outside surface of lens108.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1. A structure comprising: a semiconductor light emitting devicecomprising a light emitting layer disposed between an n-type region anda p-type region, the light emitting layer being configured to emitmonochromatic first light having a first peak wavelength; awavelength-converting material capable of absorbing the first light andemitting second light having a second peak wavelength, thewavelength-converting material being disposed in a path of the firstlight; and a filter material overlying the wavelength-convertingmaterial, wherein the filter material is configured to prevent a portionof the first light from being transmitted, and to transmit a portion ofthe first light.
 2. The structure of claim 1 wherein the filter materialis configured to absorb a portion of the first light and transmit aportion of the first light.
 3. The structure of claim 2 wherein thefilter material is configured to transmit the second light.
 4. Thestructure of claim 2 wherein the filter material is dispersed in anencapsulant overlying the semiconductor light emitting device.
 5. Thestructure of claim 2 further comprising a lens overlying thesemiconductor light emitting device.
 6. The structure of claim 5 furthercomprising a plurality of leads electrically connected to thesemiconductor light emitting device.
 7. The structure of claim 5 whereinthe filter material is coated on a surface of the lens.
 8. The structureof claim 5 wherein the filter material is incorporated into the lens. 9.The structure of claim 2 wherein the wavelength-converting material is aphosphor.
 10. The structure of claim 2 wherein the filter materialcomprises an organic dye.
 11. The structure of claim 2 wherein thefilter material does not emit visible light.
 12. The structure of claim2 wherein: the portion of transmitted first light and second lightcombine to form composite light; and the filter material is configuredsuch that the composite light corresponds to a predetermined spectrum.13. The structure of claim 1 wherein the filter material is configuredto reflect a portion of the first light and transmit a portion of thefirst light.